1. Field of the Invention
The present invention relates to an electric power unit for use in a low voltage and high current device, for example, a radar device and a CPU (central processing unit) of a computer.
2. Description of the Related Art
As is well known, according to a conventional insulated electric power unit using a transformer, in order to improve efficiency, a synchronous rectifier converter using a switching element such as an FET (field effect transistor) at its secondary side is employed. However, such a conventional system has a disadvantage such that a recovery loss due to a recovery current flowing through a body diode is large and it is difficult to obtain a high frequency.
In other words, in the case of a normal forward converter, a slope of a current at a flywheel side is found by−n·Vin/Lrwhere an input voltage is Vin, a turn ratio between a primary side and the secondary side is 1:n, and a leakage inductance at the secondary side of a transformer is Lr. That is, this slope of the current is proportional to the input voltage Vin and becomes very large because the normal inductance Lr is very small, and as a result, the recovery current depending on this slope of the current becomes very high.
In addition, in order to ensure a withstand pressure of the FET, an FET with a high on resistance must be used and this leads to lowering of an efficiency. In other words, in the case of the normal forward converter, the voltage applied to the side of the flywheel side is found byn·Vinwhere the input voltage is Vin, and the turn ratio between the primary side and the secondary side is 1:n. That is, this voltage is proportional to the input voltage Vin and in consideration of the fact that a surge voltage is further added to this voltage, the FET having a high withstand pressure is needed.
In Jpn. Pat. Appln. KOKAI Publication No. 7-337034, an art is disclosed, in which an output capacity of a MOSFET and a leakage inductance of an electric transformer are resonated, so that the need of providing an external capacitor for resonation and a reactor is cut out and then downsize the converter and the switching loss is prevented from being increased even if a switching frequency is made high by carrying out the zero-voltage switching.